The present invention relates generally to the calibration of coordinate measuring machines and more particularly to a gage utilizing objects arranged in a Latin square pattern.
The coordinate measuring machine (CMM) is a device for accurately determining the spatial relationship between two points in space. Each CMM incorporates probing components utilized to interface with the object being measured, which object is restrained on a support table. CMMs may be constructed according to one of the following classifications:
CANTILEVER (FIG. 1)--A measuring device employing three mechanical components moving along mutually perpendicular axes. Probing device 17 is attached to the component 18 with motion in the vertical direction, which in turn is attached to the first of the two horizontal components. The first 19 is fixed to the second 20 at one end 21 only, in cantilever fashion, over the support table 22.
BRIDGE--A measuring device employing three mechanical components moving along mutually perpendicular axes. The probing device is attached to the component with motion in the vertical direction, which in turn is attached to the first of the two horizontal components. The first is attached to the second at two locations forming a bridge over the support table.
COLUMN--A measuring device employing one mechanical component moving in the vertical direction to which the probing device is attached. The support table is movable in two mutually perpendicular horizontal directions.
HORIZONTAL ARM--A measuring device employing three mechanical components moving along mutually perpendicular axes. The probing device is attached to the first component movable in the horizontal direction. This component is attached to the vertical component in cantilever fashion over the support table. The vertical component is attached to the second horizontal component.
GANTRY--A measuring device employing three mechanical components moving along mutually perpendicular axes. The probing device is attached to the component moving in the vertical direction, which in turn is attached to the first horizontal component. The first component is attached to the second horizontal in two locations bridging the support table (support jacks) in a location elevated above the support table.
OTHER--Any machine utilizing axis configuration different than stated in the above categories.
The CMM is used to measure items at state of the art accuracy, i.e., tens of microinches. For these measurements to be useful the measurement capability of the CMM must be known, as a product should be accepted or rejected on the basis of its dimensions, not on the CMM's measurement error.
Measurement capability (accuracy) of a coordinate measuring machine is characterized by precision and systematic error (bias) as determined relative to a given calibrated fixture. Precision, the random (inherent) error in the measurement process of a CMM, is the inability of the measurement process of a CMM to obtain the same values (X and/or Y and/or Z) when the same point is measured more than once. The magnitude of this error may vary from one location to another within the working space of a CMM.
Systematic error (bias) is the difference between the true value of a characteristic and the average of repeated measurements on that characteristic. For a CMM, this error may be caused by error in machine geometry. Systematic error is measured by the variation introduced into the measurement process by movement along the X axis, movement along the Y axis and movement along the Z axis. This movement along the axis may contribute bias and/or random error to the measurement process. Other factors which may contribute to systematic error are environments, operators and set ups.
Knowledge of the above information provides the user of a CMM with a quantitative measure of the CMM's measurement capability. This knowledge provides a basis for determining if a CMM has the necessary accuracy to measure a product and if the decision regarding the acceptability of the product has a high probability of being correct.
There are two popular ways to evaluate accuracy and precision of a CMM. The traditional technique is to make parametric tests of the 21 sources (yaw, pitch, roll, etc.) of error that introduce inaccuracy into the measurement process. Each test is made by moving the probe along a calibrated device and comparing the measurement with the known configuration of the device. A new technique involves the use of a gage master with a plurality of gage points fixed in a plane or space, when the CMM probe is moved from point to point and the measured distance between points is compared with the known distance.
A problem with these techniques is that their measurements give an indication of the accuracy of a CMM for a limited range of measured points. The range of axis movement is limited because of the configuration and dimensions of these devices. To increase the range of axis movement over which accuracy statements would apply requires multiple movements of these devices. This movement not only introduces a non-estimable source of variation into the evaluation process but also reduces productivity by requiring additional time on the CMM.